While both "expensive" and "costly" describe something with a high price, there's a subtle difference in their usage. "Expensive" is a more general term that simply means something has a high price. "Costly," on the other hand, often implies more significant consequences beyond just the financial aspect. It suggests that the high price comes with other implications, like time investment, effort, or potential negative effects.

For example:

• "That diamond necklace is expensive." (This simply states the high price of the necklace.)
• "That diamond necklace is expensive" (This means that the diamond necklace has a high price.)
• "Taking that course was costly." (This suggests that besides the tuition fee, there were other costs associated with the course like time commitment, effort, and possible missed opportunities.)
• "Taking that course was costly." (This means that besides the tuition fee, there were other negative aspects of the course.)

Another difference is in the level of formality. "Expensive" is commonly used in everyday conversations while "costly" often appears in more formal settings.

Let's look at some more examples:

• "That car is too expensive for me." (Meaning: I cannot afford that car because of its high price.)
• "That car is too expensive for me." (Meaning: The car is too expensive to buy, therefore, I cannot afford it.)
• "That mistake was costly." (Meaning: The mistake caused considerable problems or losses, going beyond the initial monetary implications.)
• "That mistake was costly." (Meaning: That mistake had significant negative consequences.)

Remember that context is key. The best word choice depends entirely on the situation.

Happy learning!